1. Field of the Invention
The present invention relates to a method for allocating signature sequence in a CDMA system, and more particularly to a method for allocating signature sequence capable of minimizing interference among users using Park-Song (PS) sequence in a CDMA system.
2. Description of the Prior Art
Generally, a multiple access technique is a method through which a plurality of users send and receive signals at the same time, and is divided into the time division multiple access (TDMA), a frequency division multiple access (FDMA) and the code division multiple access (CDMA).
A primary object of the multiple access is that a plurality of signals do not allow to be overlapped, that is, to be jammed up in cases where a plurality of users give and take signals at the same time.
First, the TDMA of the time dividing method capable of thinking out most easily sends signals in regular time sequence. For example, a system respectively allocates a pertinent number to ten users for ten users. In this case, if any user transmits a signal, the other users interrupt transmission of signals and transmit a signal only in the case of his own order.
Second, the FDMA of the frequency dividing method is the method in which a system allocates frequencies different from each other to each user and sends the signal of the pertinent frequency. For example, a system allocates 0 to 5 kHz to one and 5 to 10 kHz to another so as to allocate different frequency bands to every user for ten users.
As described above, these TDMA and FDMA divide respectively time and frequency and use the results. That is, the TDMA divides time while sharing frequency, on the other hand the FDMA divides frequency while sharing time.
Meanwhile, the CDMA of the code dividing method is the method capable of sharing both time and frequency. Therefore, signals of a plurality of users are overlapped in both areas of time and frequency. A new method of finding out the only signal corresponding to oneself of the signals is required, and thus a system allocates codes to each user to accomplish such object.
In addition, a sequence means a queue of a number such as xe2x80x9c1, 0, 0, 1, 1, . . . , 0, 1xe2x80x9d. That is, the sequence is that the number is continued in accordance with a prescribed rule. Particularly, as mentioned above the sequence consisted of xe2x80x980xe2x80x99 and xe2x80x981xe2x80x99 is called a binary sequence, the most representative binary sequence is PN sequence widely used in the CDMA recently.
The most important characteristics of the sequence is auto-correlation and cross-correlation. The auto-correlation is a characteristics of the case of multiplying and adding one sequence and itself, and the cross-correlation is a characteristics of the case of multiplying and adding one sequence and another. A good sequence satisfies the following matters, in cases where the length of the sequence is N.
auto-correlation=N, cross-correlation=0
Meanwhile, when the system allocates k sequences having the same length to k users, the allocated sequences are called codes. As shown in FIG. 1, sequences multiplied by information data are called a spreading codes or codes in the case of sending the information data from a transmission side. Such codes are differently allocated every users in CDMA system. Thus, the most important characteristics of the CDMA start from auto-correlation and cross-correlation of the different codes. That is, if one hopes to find out a desired signal, namely, information data, the system multiplies the signal by the code of signal sending one. Then, the signal is converted into the original information data, and signals of the other users become zero by the cross-correlation characteristics.
However, there are several problems actually. First, it is difficult to make the cross-correlation zero. Second, if his own signal is arrived a little later in the auto-correlation, the signal must be become zero, but actually the signal is not zero.
Therefore, the present invention suggests a sequence superior to the prior PN sequence in view of the auto-correlation and cross-correlation, and further suggests a method applicable to CDMA.
Therefore, an object of the present invention is to provide a method for allocating signature sequence in a CDMA system capable of minimizing interference among users using Park-Song sequence of improving auto-correlation and cross-correlation characteristics.
In accordance with the present invention, this object is accomplished by providing a method for allocating signature sequence used to distinguish my own signal from the other user""s signals in a code division multiple access system comprising the steps of: obtaining a Park-Song sequence cl,k based upon an equation (1) and selecting the Park-Song sequence based upon an equation (2),                               c                      l            ,            k                          =                                            ∑                              p                =                0                                                              N                  4                                -                1                                      ⁢                                          W                                  N                  s                                lk                            ⁢                              W                V                                  v                  t                                            ⁢                              W                                  N                  4                  i2                                lp                            ⁢                              δ                ⁡                                  (                                      R                    ⁡                                          (                                                                        l                          +                          mp                                                ,                                                  N                          b                                                                    )                                                        )                                                              =                      W                          V              s                                      i              s                                                          (        1        )                                where        ,                              i            s                    =                                    Vl              ⁡                              (                                  k                  +                                      Kp                    s                                                  )                                      +                                          P                ⁡                                  (                                      β                                          p                      s                                                        )                                            ⁢                              N                s                                                    ,                              V            s                    =                      VN            s                          ,                  and          ⁢                      xe2x80x83                    ⁢                      p            s                                              xe2x80x83                                                      satisfies            ⁢                          xe2x80x83                        ⁢                          R              ⁡                              (                                                      l                    +                                          m                      ⁢                                              xe2x80x83                                            ⁢                                              p                        s                                                                              ,                                      N                    b                                                  )                                              =          0                ,                            xe2x80x83                                                                                    a                l                                  (                  k                  )                                            =                              {                                                                                                                              c                                                                                    l                              +                              N                                                        ,                                                          -                              L                                                        ,                            k                                                                          ,                                                                                                                                                                  if                            ⁢                                                          xe2x80x83                                                        ⁢                            l                                                    =                          0                                                ,                        1                        ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  L                          -                          1                                                ,                                                                                                                                                                          c                                                                                    l                              -                              L                                                        ,                            k                                                                          ,                                                                                                                                                                  if                            ⁢                                                          xe2x80x83                                                        ⁢                            l                                                    =                          L                                                ,                                                  L                          +                          1                                                ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  L                          +                                                      N                            s                                                    -                          1                                                ,                                                                                                                                                                          c                                                                                    l                              +                              N                                                        ,                                                          -                              L                                                        ,                            k                                                                          ,                                                                                                                                                                  if                            ⁢                                                          xe2x80x83                                                        ⁢                            l                                                    =                                                      L                            +                                                          N                              s                                                                                                      ,                                                  L                          +                                                      N                            s                                                    +                          1                                                ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  N                          -                          1.                                                                                                                                                                                                            =                                  c                                                            R                      ⁡                                              (                                                                              l                            +                            N                                                    ,                                                      -                            L                                                    ,                                                      N                            s                                                                          )                                                              ,                    k                                                              ,                              l                =                0                            ,              1              ,              …              ⁢                              xe2x80x83                            ,                              N                -                1                            ,                              k                =                0                            ,              1              ,              …              ⁢                              xe2x80x83                            ,                                                K                  u                                -                1.                                                                        (        2        )            
where, L is a natural number chosen based upon the number of resolvable paths and the time delays, Ku is the number of users, and N=Ns+2L is the length of the signature sequence; allocating the selected Park-Song sequence as the signature sequence to the pertinent user; and performing communications using the allocated signature sequence.